Weight painting vertex groups not entire object
AFAIK, and maybe someone will correct me on this, weight painting and assigning verts to a group are the same thing, assignment just sets weight to 1 for all assigned verts.  You may be able to assign this group to your particle system straight away.
A vertex group is basically just an extra variable attached to each vert that you can reference.  So by creating a new vertex group you essentially create a new weight paint mask where all verts have weight 0 (Blue).
I have had issues with rogue particles, not sure if I ever had a surefire solution other than what you've suggested or looking through seeds to find the least problematic one.